Why is September 11, 2001 a significant dat in us history

Because Al-Qaeda carried out the largest territory attack against the US

Its significant because two planes crashed the twin towers.

Explanation:

On September 11, 2001 a group of Islamic people hijacked four planes and planned to attack the target which is the United States. The planes were flown into the twin towers of the World Trade Center, causing them to crash and fall. This was in New York City and a third plane crashed into the pentagon in Washington D. C., the fourth plane crashed into a field in Pennsylvania. 3,000 people where killed by this crash and all of this became know as 9/11.